Tested on Mac. Folder names may differ on Windows/Linux.

You will have to do this manually for every Vehicle.

First, rightclick on Stormworks in you Steam library, and click "properties".
Then click to "local files" and "Browse Local Files"

You should see a hirachy like this:

Click on "steamapps".
Then go Into workshop>Content>573090 and into the next folder. (Name is different for everyone)
There you should see your files like this:





Now repeat the first paragraph.
Instead of "steamapps", click "Application Support" (I think %appdata% on Windows).
Now go to Stormworks>Data>vehicles.

Copy the files from the first folder you opened into the "vehicles" folder.

Open Stormworks.

Done.

(The ships might not have thumbnails ingame)